The current process for printing security documents, especially currency, is sheetfed non-heatset intaglio. Sheetfed non-heatset intaglio inks are based on oxidizable resins and alkyds and are very slow drying. Typically, one side of the currency is printed first and after 24-48 hours, the other side is printed. The typical maximum printing speed of such inks is about 75 m/min. U.S. Pat. No. 4,966,628 discloses typical non-heatset sheetfed intaglio inks suitable for printing of security documents.
The term "intaglio printing" refers to a printing process wherein a printing cylinder or printing plate carries the engraved pattern and the engraved recess is filled with printing ink to be transferred to the printing substrate in order to create the document. In this type of printing, typically a rotating engraved cylinder (usually manufactured from steel, nickel or copper and plated with chromium) is supplied with ink by one or more template inking cylinders by which a pattern of inks of different color is transferred to the printing cylinder. Any excess ink on the surface of the cylinder is then wiped off by a rotating wiper cylinder covered by a plastisol, using a dilute aqueous solution of sodium hydroxide and sulfonated castor oil as an emulsifying medium for the wiped-off excess ink. Thereafter, the printing pattern is transferred, under a pressure of up to 105 kg/cm.sup.2, to the substrate.
Recently, a heatset intaglio printing ink was developed at the laboratories of the assignee. Such ink is described in U.S. Pat. No. 5,100,934 assigned to the same assignee. The inks described in the '934 patent permitted security documents, especially currency, to be printed by heatset intaglio, thereby resulting in an improved product and in a printing process which permits the obverse of the printed document to be printed an instant after printing the face. However, the inks described in the '934 patent require relatively high heatset temperatures. While such high temperatures insure very rapid drying and high throughput rates, the high heatset temperatures also tend to produce undesirable yellowing and/or crinkling of the paper substrate. In contradistinction thereto, the heatset inks of the present invention require much lower heatset temperatures but yet result in a high quality product showing no evidence of yellowing or crinkling when printed at the same throughput rates as those utilized in intaglio printing with the '934 patent inks.
The principal requirements for a security document heatset intaglio printing ink are as follows:
(a) correct rheological properties in respect to transfer of the ink to the printing cylinder and transfer therefrom to the substrate;
(b) ability of the excess ink to be easily and quantitatively removed from the non-image areas of the die surface by the wiping cylinder (wipeability);
(c) ease of cleaning the wiping cylinder by means of a cleanser such as dilute aqueous caustic soda solution containing about 1% NaOH and 0.5% sulfonated castor oil or other surfactants;
(d) stability of the ink on the printing rollers; i.e. control of the evaporation of volatile materials during the printing process to prevent premature drying of the ink on the rollers;
(e) film-forming characteristics allowing handling of the webs carrying printed films of up to 200.mu. thickness immediately after printing;
(f) proper drying properties when printing at speeds of up to 200 m/min. with engravings of up to 200.mu. thickness;
(g) outstanding chemical and mechanical resistance of the printed document pursuant to specifications established by INTERPOL at the 5th International Conference on Currency and Counterfeiting in 1969 and by the U.S. Bureau of Engraving and Printing in BEP 88-214 (TN) .sctn. M5.
All of the foregoing requirements have been met by the ink compositions of the present invention. Indeed, the ink compositions of the present invention have been accepted by the Bureau of Engraving and Printing for use in printing currency by heatset intaglio printing.